1. Field of the Invention
The present invention relates to a method for manufacturing a magnetic recording medium, a magnetic recording medium, and a magnetic recording and reproducing apparatus.
Priority is claimed on Japanese Patent Application No. 2009-015635, filed Jan. 27, 2009, the content of which is incorporated herein by reference.
2. Description of the Related Art
The recording density of hard disk devices (HDD), which are one type of magnetic recording and reproducing apparatus, is currently increasing at an annual rate of 50% or higher, and it is expected that this trend will continue in the future. Consequently, the development of magnetic recording heads, and the development of magnetic recording media suitable for high density recording is being advanced.
Currently, as a magnetic recording medium that is installed in the commercially available magnetic recording and reproducing apparatus, a so-called perpendicular magnetic recording medium is used, in which the axis of easy magnetization within a magnetic film is mainly oriented perpendicularly. In the perpendicular magnetic recording medium, even when recording density is increased, there is an advantage in that the effects of a demagnetizing field formed in the boundary region between recording bits are small and distinct bit boundaries are formed, thus enabling noise reduction. Furthermore, in the perpendicular magnetic recording medium, since reduction in recording bit volume associated with the increase in recording density can be suppressed, there is also an advantage in that thermal stability can also be enhanced. Therefore, in recently years, a perpendicular magnetic recording medium has become of keen interest, and various medium structures suitable for perpendicular magnetic recording have been proposed.
Further, in recent years, in response to demand for magnetic recording media of higher recording density, employment of a single magnetic pole head exhibiting excellent ability to record data onto a perpendicular magnetic layer has been studied. In order to employ such a single magnetic pole head, there has been proposed a magnetic recording medium in which a layer formed of a soft magnetic material (called a “backing layer”) is provided between a perpendicular magnetic layer serving as a recording layer and a substrate, thereby enhancing the efficiency of magnetic flux flow between the single magnetic pole head and the medium.
However, when a magnetic recording medium simply provided with a backing layer is used as described above, adequate levels of various characteristics, such as the recording and reproducing characteristics during the recording/reproducing process, the resistance to thermal fluctuation and the recording resolution, cannot be attained, and thus demand for an excellent magnetic recording medium in terms of these characteristics has been increasing.
In particular, achieving both the increase in the ratio of signal to noise (S/N ratio) during reproduction and the improvement in the resistance to thermal fluctuation, which are factors important as the recording and reproducing characteristics, is essential for the future high density recording. However, these two factors are incompatible with each other, and when improving the level of one factor, the level of the other factor becomes inadequate, and thus it has become important issue to achieve high levels in both of these factors at the same time.
In order to solve the above-mentioned problem and to improve the recording and reproducing characteristics and thermal fluctuation characteristics of perpendicular magnetic recording medium, a method has been proposed for enhancing the perpendicular orientation properties of magnetic layers by using an orientation control layer and preparing a multiple magnetic layers as a continuous columnar crystal of crystal particles in each magnetic layers (for example, refer to FIG. 2 in Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. 2004-310910)).
In addition, as the orientation control layer mentioned above, for example, use of Ru has been proposed (for example, refer to Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2007-272990). The orientation control layer composed of Ru that is disclosed in Patent Document 2 has a dome-shaped convex portion on the growing surface thereof. Accordingly, by growing the crystal particles in the magnetic layers and the like on the convex portion and by promoting the separation of grown crystal particle structures and thereby isolating the crystal particles, magnetic particles are effectively grown into a columnar shape.
Further, as a method for reducing the level of magnetic interactions among the magnetic crystal particles by magnetically separating and isolating the magnetic crystal particles, a method has been proposed in which SiO2 or the like is added to the magnetic layer to form a perpendicular magnetic recording layer having a so-called granular structure, which is surrounded by the grain boundary regions where magnetic crystal particles contain a large amount of SiO2 or the like (for example, refer to Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. 2002-342908).
In addition, as a method for forming a perpendicular magnetic recording layer having a granular structure, a method has been disclosed in which a perpendicular magnetic recording layer having a granular structure is formed by using a composite-type target containing a CoCrPt alloy as well as SiO2 through a DC magnetron sputtering process in the atmosphere of mixed gas composed of argon and oxygen (for example, refer to Non-Patent Document 1 (IEEE Transactions on Magnetics, Vol. 40, No. 4, July 2004, p. 2498-2500). In Non-Patent Document 1, it has been disclosed that by conducting a reactive sputtering process in the atmosphere containing oxygen, the level of coercive force increases and the recording and reproducing characteristics also improve. Moreover, in Non-Patent Document 1, it has been disclosed that the optimal oxygen partial pressure is determined by the SiO2 concentration, and the level of optimal oxygen partial pressure increases as the SiO2 concentration reduces, and that when the oxygen concentration exceeds the optimal value and become excessive, the magnetic characteristics and recording and reproducing characteristics are considerably deteriorated.
Furthermore, a method has been proposed in which when forming a CoCrPt—SiO2-based magnetic film by a sputtering process, since the sputtering efficiency is enhanced by reducing the magnetism of a target, a Co mother alloy which is to become a ferromagnetic alloy is contained in the target in the forms of CoO and CoSi, thereby reducing the level of the target magnetism (for example, refer to Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. 2006-299401).
As described above, the demand for higher recording density with respect to the magnetic recording media has been continuously increasing, and more improvements than ever before in the recording and reproducing characteristics and the thermal fluctuation characteristics have been required for the magnetic recording media. In addition, as the extent of spacing between the head and the magnetic recording medium is decreasing, improvements in the scratch resistance of magnetic recording media have also been required so that the data can be retained even if the head and the magnetic recording medium are brought into contact.
In order to meet such demands, it is required to use a magnetic layer having a granular structure as disclosed in Non-Patent Document 1 or the like, while increasing the extent of magnetic separation and isolation of magnetic crystal particles constituting the granular structure. Further, also in terms of the method for forming a perpendicular magnetic layer exhibiting excellent electromagnetic conversion characteristics and having a granular structure, a method disclosed in Non-Patent Document 1 or the like has been adopted in which a reactive sputtering process is carried out in the atmosphere containing oxygen.
As the above-mentioned method, when forming a magnetic layer having a granular structure using a reactive sputtering process, it is necessary to increase the gas pressure in order to prevent the desorption of oxygen. However, when the present inventors and others have conducted intensive and extensive studies, it became apparent that when the gas pressure was increased during the sputter deposition process, the density of magnetic layer was reduced and the film hardness was also reduced. For this reason, it is thought that, for example, the above-mentioned problem can be solved by forming a magnetic layer having a granular structure at a low sputtering gas pressure. Here, in order to form a magnetic layer having a granular structure at a low gas pressure, for example, it is necessary to strictly control the film forming conditions and stabilize the concentration of oxygen contained in the film using a magnetron sputtering apparatus. However, since a target containing a magnetic alloy exhibits a high level of magnetic permeability and the level of magnetic flux flowing through is reduced, it becomes difficult to strictly control the sputtering conditions using a magnetron.